The elemental composition of a kg

(154 lb) male shows that ~60% is water and 16% or more is fat. Besides carbon (C), hydrogen (H), and oxygen (O), the chemical elements nitrogen (N), calcium (Ca), and phosphorus (P) are the most abundant in terms of mass (A). Most other naturally occurring elements can also be found in the human body; however, their significance is often unknown. Chemical composition changes with age. These changes are most striking during the first year of life (B). While the water content drops rapidly, fat content, protein in muscle mass, and minerals, mostly in bone, increase. The reduction in water content with age is accompanied by redistribution between body compartments (C). In the central nervous system, skin, and subcutaneous tissue, the water content drops while it increases significantly in muscle and fatty tissue. Redistribution also occurs within body compartments; intracellular water increases in all organs except skin and subcutaneous fat.

Within total body water, intra- and extracellular compartments are to be distinguished (D). Extracellular fluids can be seen as a mediator between cells and their external environment. Their composition is subject to strict homeo-static control and resembles that of a primal ocean—the primal ocean which constituted the environment of ancient, single-celled living things from which humankind ultimately evolved. Trans-cellular fluids are found in the organism's hollow spaces, like the digestive tract, bladder, or cranial fluid spaces. A simplified estimate of plasma volume can be obtained using the equation: body weight (kg) x 0.035 = plasma volume [l]. Interstitial fluid serves as a supply system to the body's tissues. It is found between the cells and makes up ~20% of body mass. Its composition is similar to a plasma ultra-filtrate with very low protein content. More than half of the total body water is found inside the cells. The intracellular space is the site of cellular metabolism. As opposed to other fluid spaces, it is not homogeneous and its composition may differ greatly between different types of cells.

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